Not long afterward, 38-year-old Kurtz applied for the 5th Special Forces Group, which was denied out of hand because his age was too advanced for Special Forces training. Kurtz continued with his ambition and even threatened to quit the armed forces, when finally his wish was granted and he was allowed to take the airborne course. Kurtz graduated in a class where he was nearly twice the age of the other trainees and was accepted into the Special Forces Training, and eventually into the 5th Special Forces Group.[citation needed]

Kurtz returned to Vietnam in 1966 with the Green Berets and was part of the hearts and minds campaign, which also included fortifying hamlets. On his next tour, Kurtz was assigned to Project GAMMA, in which he was to raise an army of Montagnards in and around the Vietnamese–Cambodian border to strike at the Viet Cong (VC) and North Vietnamese Army (NVA). Kurtz located his army, including their wives and children, at a remote abandoned Cambodian temple which Kurtz's team fortified. From their base, Kurtz led attacks on the local VC and the regular NVA in the region.[citation needed]

Kurtz employed barbaric methods not only to defeat his enemy but also to send fear. At first Military Assistance Command, Vietnam (MACV) did not object to Kurtz's tactics, especially as they proved successful. This soon changed when Kurtz allowed photographs of his atrocities to be released to the world.[citation needed]

In late 1967, after Kurtz failed to respond to MACV's repeated orders to return to Da Nang and resign his command after he ordered the summary execution of four South Vietnamese intelligence agents whom he suspected of being double agents for the Viet Cong, the MACV sent a Green Beret Captain named Richard Colby to bring Kurtz back from Cambodia. Either because he was brainwashed or because he felt a sympathy towards Kurtz's cause, Colby joined up with Kurtz instead of bringing him back to Da Nang.[citation needed]

With Colby's failure, MACV then selected Captain Benjamin L. Willard, a paratrooper and Army intelligence officer, to journey up the Nung river and kill Kurtz. Willard succeeded in his mission only because Kurtz, himself broken mentally by the savage war he had waged, wanted Willard to kill him and release him from his own suffering. Kurtz also murdered Jay "Chef" Hicks by severing his head. Before Willard killed him, Kurtz asked Willard to find Kurtz's wife and son, and explain truthfully to them what he had done in the war.[citation needed]

The movie's Kurtz is widely believed to have been modeled after Tony Poe, a highly decorated and highly unorthodox Vietnam War-era paramilitary officer from the CIA's Special Activities Division.[2] Poe was known to drop severed heads into enemy-controlled villages as a form of psychological warfare and to use human ears to record the number of enemies his indigenous troops had killed. He would send these ears back to his superiors as proof of his efforts deep inside Laos.[3][4]

However, Coppola denies that Poe was a primary influence. Coppola says the character was loosely based on Special Forces Colonel Robert B. Rheault, whose 1969 arrest for the murder of a suspected double agent generated substantial news coverage.[5]

When Brando arrived for filming in the Philippines in September 1976, he claimed he was dissatisfied with the script; Brando didn't understand why Kurtz was meant to be very thin and bald, or why the character's name was Kurtz and not something like Leighley. He claimed, "American generals don't have those kinds of names. They have flowery names, from the South. I want to be 'Colonel Leighley'." And so, for a time the name was changed under his demand.[citation needed]

When Brando showed up for filming he had put on about 40 pounds (18 kg) and forced Coppola to shoot him above the waist, making it appear that Kurtz was a 6-foot 6-inch (198 cm) giant.[6] Many of Brando's speeches were ad-libbed, with Coppola filming hours of footage of these monologues and then cutting them down to the most interesting parts.[citation needed]

Filming was put on a week-long hiatus so that Brando and Coppola could resolve their creative disputes. It is claimed that someone left Conrad's source text, which Coppola had repeatedly referred to Brando but which Brando had never read, in the houseboat where Brando was staying at the time. Brando returned to filming with his head shaved, wanting to be "Kurtz" once again; claiming it was all clear to him now that he had read Conrad's novella.[7]

Coppola first offered the role of Kurtz to Orson Welles (who had previously tried to adapt Heart of Darkness to the screen himself), but for some reason or another he declined. The documentary on the film includes the audio of a radio version that Welles did during his prime.

The original script by John Milius climaxes with Kurtz fighting off an attack by U.S. Army helicopters, firing a machine gun while exclaiming to Willard, "I can feel the power in my loins!" Francis Ford Coppola thought this was absurd.